Unitil recently I shot youth football( soccer that is - not the version played by men with odd shaped balls;)) with a basic Olympus E420 and a 70-300 lens. Results were fine. One month ago purchased Canon 7d with 50-250 and results are good. Now been tempted with an offer to shoot first team action and used 50-250 for first shoot. Results still good but plan to get faster lens.

In terms of budget I can afford either Canon 70-200 f4 (c£500) or Sigma 70-200 f2.8 (c£650). I may also add 1.4x and/or 2x.

Well, since you're OK with f4 but you'd like more reach, I would say save up for option c) Sigma 100-300mm. If that's absolutely out of the question, since your other two choices cover the same focal length range (actually less) as your current 50-250mm, I would say go for the largest aperture difference. i.e. Sigma f2.8

I use both the sigma 70-200 2.8, and sigma 100-300 f4 to shoot my kids playing rugby league, actually I haven't used the 300 with the kids games yet as I purchased it just after the end of the season, but I did hire a sigma 120-300 2.8 a couple of times, and for me 300mm was enough, the 200 was great with the youngest son as they were still on the half fields, but it was a bit short with my eldest as they were playing on full fields, 2.8 does come in handy when the light gets low/bad, but f4 was still OK, that's why I sold my 100-400 and bought the 100-300, for me, the extra stop was worth more than IS and an extra 100mm.